The present invention relates to a wavelength-variable semiconductor laser and to an integrated optical device in which such a laser is built.
A distributed feedback semiconductor laser diode (DFB LD) having a diffraction grating built-in, because it oscillates in a single longitudinal mode (SLM), seems to promise effective application to long-distance large-capacity optical fiber communication and future systems of optical coherent transmission. In an optical heterodyne communication system, the local oscillator of light has to follow the light signal while maintaining a constant wavelength difference in order to provide beat signals. Therefore, an SLM laser for use as the local oscillator requires an ability to exercise tuning, in particular continuous fine tuning, of the oscillation wavelength.
Such wavelength-variable SLM semiconductor lasers include the facet phase-tunable DFB LD, as referred to by Kitamura et al., "Phase Tunable DFB-DC-PBH LD", 1984 NATIONAL CONVENTION (RECORD) of ELECTRONICS AND COMMUNICATION ENGINEERS OF JAPAN, Vol. 4, Paper No. 1024 (in Japanese).
FIG. 1 is a schematic diagram of this facet phase-tunable DFB LD, which consists of a laser region 2 in which a diffraction grating 1 is formed and a tuning region 3. The facet 4 of the tuning region 3 is formed by cleaving, and has a light reflectivity of about 30%. In this facet phase-tunable DFB LD, an electric current is injected into the tuning region 3 to alter the refractive index of this region, so that the light path length is equivalently varied to tune the phase of the light beam on the border between the laser region 2 and the tuning region 3 as the reflected light beam returns from the facet 4 and thereby to tune the oscillation wavelength.
The current injection into the tuning region 3 causes the refractive index of this region to vary due to the plasma effect. Accordingly, the phase of the light beam which enters the tuning region 3 from the laser region 2 is changed and returns reflected by the end face 4. This behavior is the same as what would take place if, in a DFB LD element wherein the tuning region 3 of FIG. 1 were absent and only a DFB region 2 were present, the position (the phase of the diffraction grating) in which the diffraction grating is cut off at the right end of the laser region 2 were varied. FIG. 2 shows a curve representing the wavelength-dependence of the oscillation threshold gain in this case. When the position in which the diffraction grating is cut off varies, the oscillation mode corresponding to that position shifts in the sequence of A B.fwdarw.C D.fwdarw.A. (See, for example, K. Utaka et al., "Effect of Mirror Facets on Lasing Characteristics of Distributed Feedback InGaAsP/InP Laser Diodes at 1.5 .mu.m Range", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-20, No. 3, March 1984, pp. 236-245.) If a current is injected into the tuning region of a facet phase tunable laser to control the quantity of the phase shift of the light beam reflecting from the facet 4, the oscillation mode will shift on the oscillation threshold control curve in the sequence of A B.fwdarw.C D.fwdarw.A as indicated by the arrows in FIG. 2, so that continuous wavelength tuning can be achieved between B and C and between D and A. However, there also emerges between point A and point B, where a Bragg wavelength lies, a region where no oscillation mode exists at all (hereinafter referred to as a stopband). Therefore, in this facet phase tunable DFB LD, selective oscillation of either one of the two modes, i.e. one existing between A and D and the other between B and C, is possible by controlling the quantity of the phase shift in the tuning region 3. In this facet phase tunable DFB LD, however, the presence of the stopband makes impossible continuous wavelength tuning between A and B and, although fine tuning of the oscillation wavelength between A and D or B and C is possible, the finely tunable range is less than 1 .ANG., so that the application of such a DFB LD to the optical heterodyne system or the like is impossible.